d- and f-Block Elements · Lesson 3 of 12
Oxidation States, Oxides and Halides
“Use d-electron bookkeeping and ligand bonding to explain variable oxidation states and oxide acidity.”
• Assign metal oxidation states in oxides, oxoanions, halides and carbonyls. • Explain the broad rise and fall of accessible oxidation states across 3d. • Relate oxygen and fluorine bonding to high-state stabilisation. • Compare acidic, basic and amphoteric oxides and recognise mixed-valence compounds.
Several electron counts can lead to bonded compounds
Iron occurs in both FeCl₂ and FeCl₃; manganese appears in MnO, MnO₂ and KMnO₄. Transition-metal s and d valence levels have sufficiently similar energies that different numbers of electrons can be involved in bonding. Whether a particular state is stable depends on what it is bonded to, the surrounding solvent, and competing reactions. Oxidation state is a formal assignment of electrons to atoms under agreed rules, not always an actual ionic charge.
| Element | Oxidation states represented in common/simple chemistry | Broad interpretation |
|---|---|---|
| Sc | +3 | Few valence electrons; stable d⁰ ion |
| Ti | +2, +3, +4 | High state +4 often favoured |
| V | +2, +3, +4, +5 | Several adjacent states accessible |
| Cr | +2, +3, +4, +5, +6 | +3 and +6 especially familiar |
| Mn | +2 through +7 | Largest range in this row |
| Fe | +2, +3; +6 in ferrate | High states require suitable bonding/medium |
| Co | +2, +3; +4 in suitable compounds | Ligands strongly affect stability |
| Ni | Mainly +2; +3,+4 in suitable compounds | High states less broadly stable |
| Cu | Mainly +1,+2 | d¹⁰/d⁹ chemistry |
| Zn | Normally +2 | Outer 4s loss leaves d¹⁰ |
The maximum well-established simple oxide state rises from Sc(+3) through Ti(+4), V(+5), Cr(+6) to Mn(+7). Beyond manganese, very high states become much less broadly stable. Early metals have too few valence electrons to reach many positive states; late metals would pay increasingly large costs to remove or strongly involve more tightly held d electrons. Sc is a useful counterexample to the claim that all transition elements must show several familiar oxidation states: ordinary Sc chemistry is overwhelmingly +3.
Adjacent states often differ by one unit, for example V(II), V(III), V(IV), V(V). Main-group variable states frequently differ by two, such as Sn(II)/Sn(IV), though this is a contrast in common patterns rather than an exceptionless law. Among heavier d metals, high states can be more persistent: Mo(VI) and W(VI) oxides are less strongly oxidising than acidic Cr(VI). This contrasts with the familiar tendency for some heavier p-block elements to favour lower states.
Oxygen and fluorine make high states possible
Removing electrons from a metal requires energy. Strong metal–ligand bonding and lattice formation can compensate. Fluorine’s small size and strong attraction for electrons support high-state fluorides such as VF₅, CrF₆ and CoF₃. Oxygen can make strong multiple-bonding interactions in oxides and oxoanions: Mn reaches +7 in Mn₂O₇ and MnO₄⁻, whereas its highest simple fluoride in this treatment is MnF₄. High oxidation state does not require a bare M⁷⁺ ion to float freely in water.
Problem
Assign V in VO₂⁺ and VO²⁺, Cr in CrO₄²⁻, and Mn in MnO₄²⁻ and MnO₄⁻.
- 1.For VO₂⁺ write x+2(−2)=+1, so x=+5. For VO²⁺ write x−2=+2, so x=+4. The position of the 2 changes its meaning.
- 2.For CrO₄²⁻, x−8=−2 gives +6. For MnO₄²⁻ the same calculation gives +6; for MnO₄⁻ it gives +7.
- 3.Formal d counts follow group minus oxidation state: V(V) d⁰, V(IV) d¹, Cr(VI) d⁰, Mn(VI) d¹ and Mn(VII) d⁰.
| State | Oxide examples | Interpretive point |
|---|---|---|
| +1 | Cu₂O | Copper(I) |
| +2 | TiO, VO, MnO, FeO, CoO, NiO, CuO, ZnO | Low-state oxides usually more ionic/basic |
| +3 | Sc₂O₃, Ti₂O₃, V₂O₃, Cr₂O₃, Mn₂O₃, Fe₂O₃ | Cr₂O₃ is amphoteric |
| +4 | TiO₂, VO₂, CrO₂, MnO₂ | Intermediate chemistry depends on metal |
| +5 | V₂O₅ | Amphoteric, with substantial acidic character |
| +6 | CrO₃ | Acidic oxide |
| +7 | Mn₂O₇ | Covalent acidic oxide |
Why oxidation state changes oxide acidity
A low-charge metal cation binds the oxide ion comparatively ionically; oxide accepts protons, so the solid tends to react with acid. Increasing metal oxidation state increases polarising power and covalent character. High-state oxides often act as acid anhydrides and form oxoanions in base. Thus MnO is basic whereas Mn₂O₇ is acidic. Cr₂O₃ reacts with both suitable acids and strong bases and is amphoteric; V₂O₅ also shows amphoteric behaviour while being predominantly acidic. Structure and medium still matter, so these are explained trends, not a universal label assigned from oxidation number alone.
For vanadium, V₂O₃ is basic; VO₂ (also written V₂O₄) is less basic and gives vanadyl VO²⁺ in acid. V₂O₅ gives dioxovanadium(V), VO₂⁺, in acid and vanadate species in base. A convenient formal strongly alkaline equation is V₂O₅+6OH⁻→2VO₄³⁻+3H₂O; actual vanadate speciation changes with pH and concentration. CrO₃ with water gives chromic-acid species, while Mn₂O₇+H₂O→2HMnO₄ represents its acid-anhydride relationship. These are chemical representations, not suggestions for handling the highly reactive oxide.
Halide exceptions are chemical evidence
Not every metal–halogen combination preserves a chosen oxidation state. Cu(II) iodide is unstable because Cu²⁺ oxidises I⁻ to I₂ while becoming Cu(I), which precipitates as CuI: 2Cu²⁺+4I⁻→2CuI(s)+I₂. For the familiar halides, TiX₄ occurs for X=F,Cl,Br,I; VF₅ supports V(V); V(IV) halides are represented by F,Cl,Br; CrF₆ and CrF₅ are fluorides of high states. MnF₄ and MnF₃ contrast with the much broader MnX₂ family. FeX₃ is familiar for F,Cl,Br and CoF₃ illustrates fluorine stabilisation of Co(III). CuX₂ is familiar for F,Cl,Br, and Cu(I) chloride, bromide and iodide are well known.
Low oxidation states are supported by ligands such as CO that accept electron density from the metal as well as donate to it. CO is neutral in formal oxidation-state counting, so Ni in Ni(CO)₄ and Fe in Fe(CO)₅ is zero. This illustrates why oxidation state cannot be predicted from a metal’s usual aqueous cation alone.
Problem
Interpret Fe₃O₄, Mn₃O₄ and Co₃O₄.
- 1.Four oxide ions contribute −8, so the three metal atoms contribute +8 in total: the mean oxidation state is +8/3.
- 2.The conventional mixed-valence description is one M(II) and two M(III) per formula unit, corresponding formally to MO·M₂O₃.
- 3.The average +8/3 is bookkeeping over different sites, not a claim that all metal atoms are identical free ions of fractional charge.
Quick Quiz
Quiz
What is the metal oxidation state in VO₂⁺?
Explanation: Two O atoms contribute −4; the ion charge is +1, so V is +5.
Why can oxygen support Mn(VII) in an oxide?
Explanation: Bonding compensation allows a high formal oxidation state.
Which is an amphoteric oxide here?
Explanation: Cr₂O₃ reacts with both suitable acids and strong bases.
Why does Cu²⁺ with iodide give CuI and iodine?
Explanation: Cu changes +2 to +1 while I changes −1 to 0.
What is Fe’s formal state in Fe(CO)₅?
Explanation: CO is neutral and the whole molecule is neutral.
Practice Problems
- Calculate oxidation states in Sc₂O₃, TiO₂, V₂O₅, CrO₃ and Mn₂O₇.
- Explain why oxidation-state variability is widest around manganese.
- Why is saying “d³ always gives +3” incorrect?
- Compare the acidity of MnO, Cr₂O₃ and CrO₃.
- Balance copper(II) plus iodide to give copper(I) iodide and iodine.
- A metal atom is described only as d⁵, d⁸ or d⁴. Can its stable state be uniquely predicted?
Use O=−2 and zero total charge. The states are +3,+4,+5,+6,+7. This sequence matches the early-row group numbers; it is not a claim that all subsequent metals have equally stable oxides at their group numbers.
Key Takeaways
• Transition-metal states are formal electron assignments influenced by their chemical environment. • Adjacent oxidation states are common and the widest first-row range occurs near Mn. • Strong oxygen and fluorine bonding stabilises high formal states. • Oxide acidity generally rises with metal oxidation state, with amphoteric intermediate cases. • VO₂⁺ and VO²⁺ denote different oxygen counts and metal states. • Mixed-valence oxides have an average state that need not belong to each individual site. • Neutral carbonyl ligands support zero-state metal compounds.